1. Field of the Invention
The present invention relates to a toner for use in electrophotography and a preparing method thereof. In addition, the present invention also relates to a developer, an image forming method, an image forming apparatus, and a process cartridge using the toner.
2. Discussion of the Background
In electrophotography, an image is typically formed as follows:
(1) charging the surface of a photoreceptor serving as an image bearing member by electric discharge (i.e., charging process);
(2) irradiating the charged surface of the photoreceptor to form an electrostatic latent image thereon (i.e., irradiating process);
(3) developing the electrostatic latent image formed on the photoreceptor by supplying a toner thereto to form a toner image thereon (i.e., developing process);
(4) transferring the toner image formed on the photoreceptor onto the surface of a transfer medium (i.e., transfer process);
(5) fixing the toner image formed on the surface of the transfer medium thereto (i.e., fixing process); and
(6) removing toner particles remaining on the surface of the image bearing member after the transfer process (i.e., cleaning process).
Electrophotographic full-color image forming apparatuses have been widely used recently. Since digital images can be easily obtained, printed images thereof are required to have much higher definition. In attempting to improve resolution and gradient of the printed images, a toner, which visualize a latent image, is improved to be much more spherical in shape and smaller in size. Since pulverized toners are limited in shape and size, polymerized toners such as suspension polymerization toners, emulsion aggregation toners, and dispersion polymerization toners, have been used recently.
As disclosed in Japanese Patent No. (hereinafter referred to as JP) 3486707, a polymerized toner has an advantage in producing high definition images. However, such a toner has a drawback of increasing a non-electric adherence between a photoreceptor because of having a small particle diameter, and therefore toner particles tend to remain on the photoreceptor and form a toner film thereon after the transfer process. The toner has another drawback of passing through a cleaning blade because of having a spherical shape.
In electrophotography, a toner is required to have separativeness (hereinafter referred to as hot offset resistance) in that the toner is separated from a heating member such as a heat roller in a fixing process using a contact heating method. In attempting to improve hot offset resistance, JP 3640918 discloses a toner including a modified polyester resin prepared by reacting a precursor of the polyester resin, prepared by a dissolution suspension method.
In recent attempt to improve energy conservation in electrophotography, a toner is required to have low-temperature fixability in that the toner can sufficiently melt even under low fixing temperatures. For example, a toner having a core-shell structure, in which the core having low-temperature fixability is covered with the shell having thermal resistance, is proposed. Such a toner having a core-shell structure can be prepared by a phase separation method, a salting-out aggregation method, an in-situ polymerization method, a spray dry method, an interfacial polymerization method, etc.
JP 3786107 discloses a toner having a core-shell structure in which the core formed by aggregating and fusing a first particulate resin and a colorant is covered with the shell formed by aggregating and fusing a second particulate resin which is stably dispersed in an aqueous medium.
Published unexamined Japanese Patent Application No. (hereinafter referred to as JP-A) 2004-004506 discloses a toner having a core-shell structure prepared as follows. Droplets of a monomer for preparing the shell, which have a smaller average particle diameter than core particles, are added to a suspension liquid containing core particles, and then the mixture is subjected to a dispersion treatment using an ultrasonic emulsifier. A water-soluble polymerization initiator is further added thereto so that the monomer is polymerized at the surface of the core particles.
JP 3305998 discloses a toner having a core-shell structure in which the core including a colorant and a thermoplastic resin prepared by polymerizing a monomer is covered with the shell formed by seed polymerizing a second monomer.
JP-A 02-259657 discloses a method of preparing an encapsulated toner. In this method, cross-linked resin particles prepared by a suspension-polymerization are added to a solution including a monomer for encapsulation, and then a poor solvent (in which the monomer is not dissolved) is added thereto.
Since the above-mentioned methods include two processes of preparing a core particle and forming a shell, these methods tend to be complicated. Since the shell is formed by a polymerization reaction at a time of preparing the core particle, it is difficult to grasp the resin properties of the shell before the resultant toner is prepared. There is a possibility that the monomer for preparing the shell comes into and/or remains inside the core particle. When the monomer is polymerized inside the core particle, low-temperature fixability of the core particle deteriorates.
The same can be said for a toner disclosed in JP-A 2005-301261 having a shell including a modified polyester resin prepared by reacting a polyester precursor, which is prepared by a dissolution suspension method. It is difficult to grasp the resin properties of the shell before the resultant toner is prepared. In addition, the kind of the modified resin that can be used is limited.
In general, electrophotographic full-color images have poor color reproducibility compared with those produced by silver halide photography and printing, and the image quality thereof does not reach the level satisfying the users' eyes.
In order to raise the electrophotographic full-color image quality to the level that of silver halide photography and printing, a toner for use in electrophotography needs to include a colorant having good color reproducibility and high coloring power. Conventionally, pigments are generally used as colorants. The pigments have better light resistance and heat resistance compared with dyes, but most of the pigments tend to have poor dispersibility in the resultant toner, and therefore the kind of the pigment that can be use for the toner is limited. Therefore, full-color images produced with such a conventional toner have poor color reproducibility, coloring power, transparency, image definition, and image density.
In attempting to solve the above problems, a technique in which the content of a colorant is increased in a toner so that the coloring power thereof is increased is proposed. However, when the content of the colorant is increased, the number of the colorant particles present at the surface of the toner particles increases, and therefore chargeability, developability, and transferability of the toner deteriorate.
JP-A 11-231572 discloses a toner in which a colorant is highly dispersed with a polymer dispersant and a synergist which interacts with both the colorant and the polymer dispersant. However, the synergist tends to deteriorate chargeability, developability, and transferability of the resultant toner.
Toners for use in electrophotography are simultaneously required to have good ability to produce high definition images, color reproducibility, transferability, fixability, preservability, and cleanability, each of which is difficult to satisfy at the same time. Various attempts have been made to respond to the above requirement for a long period of the time. It is considered that the above requirement can be achieved by a toner having an inner layer and an outer layer, wherein the function of the inner layer (e.g., coloring ability, low-temperature fixability) and that of the outer layer (e.g., transferability, fluidity, cleanability, toner filming resistance) are separated.
In attempting to simultaneously improve both thermostable preservability and fixability, a toner having a core-shell structure in which the outer layer of the toner is formed of a shell having high heat resistance is proposed. However, the thermostable preservability of the toner depends on the thickness of the shell. When the thickness is too large, the shell inhibits the melting of the core when fixed, and therefore a wax (i.e., a release agent for separating from a fixing roller) cannot sufficiently exude therefrom. When the thickness is too small, the shell cannot sufficiently exert its protection effect.
JP-A 2006-065001 discloses a toner having a core-shell structure in which the shell includes a cellulose derivative having a micro-porous structure. When the toner is fixed, the core having a low glass-transition temperature expands and spreads out the shell. Since the shell has a micro-porous structure, the expansion force of the core concentrates on holes of the shell, and therefore the shell is easily cracked and the core is easily exposed. It is described therein that such a toner has both thermostable preservability and fixability. However, the shell inhibits the melting of the core when fixed, and therefore the fixability of the toner is not sufficient.
The final shape of the above-mentioned toners having a core-shell structure basically depends on that of the core. The shell does not influence the toner shape, and is not required to impart transferability and cleanability to the toner by changing the toner shape. However, cleanability, fluidity, and transferability of a toner can be improved when the toner has an irregular shape reasonably far from a true spherical shape.
In attempting to improve cleanability, JP-A 2005-274964 and JPs 2844795 and 2762507 have disclosed toners having core-shell structures in which the shells have projections. Since these toners are prepared by methods including extra processes of forming the projections, there is a drawback that the manufacturing cost thereof increases.
JPs 2750853 and 2838410 have disclosed toners in which small resin particles are mechanically buried in mother resin particles. The small resin particles form a discontinuous independent resin phase on the mother resin particles, and the resin phase impart good thermostable preservability to the resultant toner without deteriorating fixability of the mother resin particles. However, when projections are formed by mechanical or thermal methods as mentioned above, the projections easily peel off from the surface of the mother particles upon application of mechanical stress and cannot be sufficiently fixed thereto.
JP-A 2005-17773 discloses a particle in which projections are chemically bounded to the surface of a mother particle. In particular, the mother particle having functional groups is immersed in or mixed with an organic compound such as a carbodiimide compound, an epoxy compound, and an oxazoline compound so that projections are chemically bounded to the surface of the mother particle. In this case, the particle has a stiff structure because the projections are chemically bounded to the mother particle and impregnated therein. However, there is a drawback that the particle is prepared by a complicated method including a process of forming the mother particle, and an extra process of forming projections on the mother particle by mixing with a medium in which an organic compound having a reactive group capable of reacting with functional groups of the mother particle is dissolved or dispersed therein. No mention is made of whether the particle has functions of a toner (such as chargeability and thermal property). In addition, no mention is made of how the size of the projection (i.e., the degree of the exposure of the mother particle) effects on toner properties.
Because of these reasons, a needs exist for a toner which can produce high-quality full-color images comparable to those produced by silver halide photography or printing.